Just to sum up and add to what others above have written:
After for instance 20 minutes, you get a negative iodine reading, it only means that the gelatinized and soluble starches has been converted. Iodine does not show starch positive on starches that has not come into solution.
Further, if this sample was taken at 65C, then you ramp up to 73C, you will most likely read a positive test at that temperature, because some starches are not soluble before a given temperature is reached. So there's more starches to convert at 65C, which will not convert unless you turn up the heat.
There's a lot of enzymatic activity going on after conversion is done. If you mash at 65C, tightly milled, you'll have conversion pretty quickly, but you will not get a high fermentable wort for instance, since the Beta amylases haven't had enough time to do their job, they work slower, because they work on the fruits of the Alpha Amylase.
So trying to do a Saison with a non diastaticus strain or no sugar, and mashing at 65C until conversion is complete, will not make a dry saison like a 2hr mash at that temperature would yield.
It's also possible just just hold back a percentage of the grist, and add it to the mash after a while if you're mashing for a long time, to get fresh enzymes into the mash.